clapfig

Rich, layered configuration for Rust applications. Define a struct, point at your files, and go.

clapfig discovers, merges, and manages configuration from multiple sources — config files, environment variables, and programmatic overrides — through a pure Rust builder API. The core library has no dependency on any CLI framework: you can use it in GUI apps, servers, or with any argument parser. For clap users, an optional adapter provides drop-in config gen|list|get|set subcommands with zero boilerplate.

Built on confique for struct-driven defaults and commented template generation.

Features

Core (always available, no CLI framework needed):

• Struct as source of truth — define settings as a Rust struct with defaults and /// doc comments
• Layered merge — defaults < config files < env vars < overrides, every layer sparse
• Multi-path file search — platform config dir, home, cwd, ancestor walk, or any path, in precedence order
• Search modes — merge all found configs (layered overrides) or use the first match ("find my config")
• Ancestor walk — SearchPath::Ancestors walks up from cwd to find project configs, with configurable boundary (.git, filesystem root)
• Prefix-based env vars — MYAPP__DATABASE__URL maps to database.url automatically
• Strict mode — unknown keys in config files error with file path, key name, and line number (on by default)
• Template generation — emit a commented sample config derived from the struct's doc comments
• Persistence — patch values in place, preserving file comments

Clap adapter (clap feature, on by default):

• Config subcommand — drop-in config gen|get|set|list commands for clap
• Auto-matching overrides — map clap args to config keys by name in one call

Quick Start

Define your config with confique's Config derive:

use confique::Config;
use serde::{Serialize, Deserialize};

#[derive(Config, Serialize, Deserialize, Debug)]
pub struct AppConfig {
    /// The host address to bind to.
    #[config(default = "127.0.0.1")]
    pub host: String,

    /// The port number.
    #[config(default = 8080)]
    pub port: u16,

    /// Database settings.
    #[config(nested)]
    pub database: DbConfig,
}

#[derive(Config, Serialize, Deserialize, Debug)]
pub struct DbConfig {
    /// Connection string URL.
    pub url: Option<String>,

    /// Connection pool size.
    #[config(default = 10)]
    pub pool_size: usize,
}

Load it in one line:

use clapfig::Clapfig;

fn main() -> anyhow::Result<()> {
    let config: AppConfig = Clapfig::builder()
        .app_name("myapp")
        .load()?;

    println!("Listening on {}:{}", config.host, config.port);
    Ok(())
}

That app_name("myapp") call sets sensible defaults:

• Searches for myapp.toml in the platform config directory
• Merges env vars prefixed with MYAPP__
• Fills in #[config(default)] values for anything not provided

Setup

Defaults from app_name

Builder methods

use clapfig::{Clapfig, SearchPath};

let config: AppConfig = Clapfig::builder()
    // Required — sets defaults for file_name, search_paths, env_prefix
    .app_name("myapp")

    // Optional overrides
    .file_name("settings.toml")                                   // override config file name
    .search_paths(vec![SearchPath::Platform, SearchPath::Cwd])    // replace default search paths
    .add_search_path(SearchPath::Cwd)                             // append a path without replacing
    .env_prefix("MY_APP")                                         // override env var prefix
    .no_env()                                                     // disable env var loading entirely
    .strict(false)                                                // disable strict mode (allow unknown keys)
    .cli_override("host", some_value)                             // override a key from a CLI arg
    .load()?;

Search Paths, Modes, and Persistence

Config file handling has three orthogonal axes on the builder:

• Discovery (.search_paths()) — where to look. Paths listed in priority-ascending order (last = highest).
• Resolution (.search_mode()) — Merge (default: deep-merge all found files) or FirstMatch (use the single highest-priority file).
• Persistence (.persist_path()) — where config set writes. Explicit, independent of search paths.

use clapfig::{Clapfig, SearchPath, SearchMode, Boundary};

// Layered: global + local (default Merge mode)
let config: AppConfig = Clapfig::builder()
    .app_name("myapp")
    .search_paths(vec![SearchPath::Platform, SearchPath::Cwd])
    .persist_path(SearchPath::Home(".myapp"))
    .load()?;

// Find nearest project config (walk up to .git, use first match)
let config: AppConfig = Clapfig::builder()
    .app_name("mytool")
    .search_paths(vec![SearchPath::Ancestors(Boundary::Marker(".git"))])
    .search_mode(SearchMode::FirstMatch)
    .load()?;

Available SearchPath variants: Platform, Home(".myapp"), Cwd, Path(PathBuf), Ancestors(Boundary).

Ancestors walks up from cwd, expanding inline into multiple directories (shallowest first, cwd last = highest priority). Boundary::Root walks to the filesystem root; Boundary::Marker(".git") stops at the directory containing the marker (inclusive).

Missing files are silently skipped. See the types module docs for the full conceptual guide and use-case examples.

Strict Mode

Strict mode is on by default. If a config file contains a key that doesn't match any field in your struct, loading fails with a clear error including the file path, key name, and line number:

Unknown key 'typo_key' in /home/user/.config/myapp/myapp.toml (line 5)

Disable it with .strict(false) if you want to allow extra keys.

Environment Variables

With env prefix MYAPP, variables map via double-underscore nesting:

__ (double underscore) separates nesting levels. Single _ within a segment is literal (part of the field name).

Disable env loading entirely with .no_env().

Programmatic Overrides

The cli_override and cli_overrides_from methods on the builder work with any value source — they are not clap-specific despite the name. Use them to inject overrides from CLI args, GUI inputs, HTTP requests, or anything else.

Auto-matching

If your override source derives Serialize, cli_overrides_from auto-matches fields by name against config keys:

use clap::Parser;
use serde::Serialize;

#[derive(Parser, Serialize)]
struct Cli {
    #[command(subcommand)]
    #[serde(skip)]
    command: Commands,

    #[arg(long)]
    host: Option<String>,

    #[arg(long)]
    port: Option<i64>,

    #[arg(long)]
    db_url: Option<String>,
}

fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();

    let config: AppConfig = Clapfig::builder()
        .app_name("myapp")
        .cli_overrides_from(&cli)                // auto-matches host, port
        .cli_override("database.url", cli.db_url) // manual: name doesn't match
        .load()?;

    Ok(())
}

cli_overrides_from(source) serializes the source, skips None values, and keeps only keys that match a config field. Non-matching fields (command, db_url) are silently ignored. Works with any Serialize type — structs, HashMaps, etc.

Manual overrides

For fields where the CLI name differs from the config key, use cli_override:

.cli_override("database.url", cli.db_url)

cli_override(key, value) takes Option<V> where V: Into<toml::Value> — None is silently skipped. Dot notation addresses nested keys.

Both methods compose freely and push to the same override list. Later calls take precedence.

Supported value types: String, &str, i64, i32, i8, u8, u32, f64, f32, bool.

Clap Adapter (optional)

Requires the clap Cargo feature (enabled by default). To use clapfig without clap:

clapfig = { version = "...", default-features = false }

Config Subcommand

Add config management to your CLI by nesting clapfig::ConfigArgs:

use clap::Subcommand;
use clapfig::{Clapfig, ConfigArgs, ConfigResult};

#[derive(Subcommand)]
enum Commands {
    /// Run the application
    Run,
    /// Manage configuration
    Config(ConfigArgs),
}

fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();

    match cli.command {
        Commands::Config(args) => {
            let action = args.into_action();
            let result = Clapfig::builder::<AppConfig>()
                .app_name("myapp")
                .handle(&action)?;
            match result {
                ConfigResult::Template(t) => print!("{t}"),
                ConfigResult::KeyValue { key, value, doc } => {
                    for line in &doc { println!("# {line}"); }
                    println!("{key} = {value}");
                }
                ConfigResult::ValueSet { key, value } => {
                    println!("Set {key} = {value}");
                }
            }
        }
        Commands::Run => {
            let config: AppConfig = Clapfig::builder()
                .app_name("myapp")
                .cli_override("host", cli.host)
                .cli_override("port", cli.port)
                .load()?;
            println!("Listening on {}:{}", config.host, config.port);
        }
    }

    Ok(())
}

This gives your users:

myapp config gen                    # print commented sample config to stdout
myapp config gen -o myapp.toml      # write to file
myapp config get database.url       # print the resolved value of a key
myapp config set port 3000          # persist a value to the user's config file

Template Generation

config gen produces a commented TOML file derived from your struct's /// doc comments:

# The host address to bind to.
# Default: "127.0.0.1"
#host = "127.0.0.1"

# The port number.
# Default: 8080
#port = 8080

[database]
# Connection string URL.
#url =

# Connection pool size.
# Default: 10
#pool_size = 10

The template stays in sync with code — it's generated from the same struct. Change a doc comment or a default, the template reflects it.

Layer Precedence

Compiled defaults     #[config(default = ...)]
       ↑ overridden by
Config files          search paths in order, later paths win
       ↑ overridden by
Environment vars      MYAPP__KEY
       ↑ overridden by
Overrides             .cli_override()

Every layer is sparse. You only specify the keys you want to override. Unset keys fall through to the next layer down.

Persistence

config set <key> <value> writes to the path set via .persist_path() on the builder.

• The persist path is independent of search paths — you choose where writes go explicitly.
• If the file exists, the key is patched in place using toml_edit, preserving existing comments and formatting.
• If the file doesn't exist, a fresh config is created from the generated template with the target key set.
• If .persist_path() is not set, config set returns ClapfigError::NoPersistPath.

Demo Application

The repo includes a runnable example that exercises every clapfig feature — nested config structs, file search paths, env vars, CLI overrides, and the config subcommand. It's a good starting point for integration and for ad-hoc testing.

# Print all resolved values (default color: yellow)
cargo run --example clapfig_demo -- echo

# Override via env var
CLAPFIG_DEMO__DISPLAY__COLOR=red cargo run --example clapfig_demo -- echo

# Override via CLI flag
cargo run --example clapfig_demo -- --color blue --port 8080 echo

# Config subcommands
cargo run --example clapfig_demo -- config gen
cargo run --example clapfig_demo -- config list
cargo run --example clapfig_demo -- config get server.port

See examples/clapfig_demo/ for the full source.

Full Example (with clap)

use clap::{Parser, Subcommand};
use confique::Config;
use serde::{Serialize, Deserialize};
use clapfig::{Clapfig, ConfigArgs, ConfigResult, SearchPath};

// -- Config struct --

#[derive(Config, Serialize, Deserialize, Debug)]
pub struct AppConfig {
    /// The host address to bind to.
    #[config(default = "127.0.0.1")]
    pub host: String,

    /// The port number.
    #[config(default = 8080)]
    pub port: u16,

    /// Database settings.
    #[config(nested)]
    pub database: DbConfig,
}

#[derive(Config, Serialize, Deserialize, Debug)]
pub struct DbConfig {
    /// Connection string URL.
    pub url: Option<String>,

    /// Connection pool size.
    #[config(default = 10)]
    pub pool_size: usize,
}

// -- CLI --

#[derive(Parser, Serialize)]
#[command(name = "myapp")]
struct Cli {
    #[command(subcommand)]
    #[serde(skip)]
    command: Commands,

    #[arg(long, global = true)]
    host: Option<String>,

    #[arg(long, global = true)]
    port: Option<i64>,
}

#[derive(Subcommand)]
enum Commands {
    Run,
    Config(ConfigArgs),
}

// -- Main --

fn main() -> anyhow::Result<()> {
    let cli = Cli::parse();

    match cli.command {
        Commands::Config(args) => {
            Clapfig::builder::<AppConfig>()
                .app_name("myapp")
                .add_search_path(SearchPath::Cwd)
                .persist_path(SearchPath::Cwd)
                .handle_and_print(&args.into_action())?;
        }
        Commands::Run => {
            let config: AppConfig = Clapfig::builder()
                .app_name("myapp")
                .add_search_path(SearchPath::Cwd)
                .cli_overrides_from(&cli)
                .load()?;

            println!("Listening on {}:{}", config.host, config.port);
            if let Some(url) = &config.database.url {
                println!("Database: {}", url);
            }
        }
    }

    Ok(())
}